This invention relates generally to electrical apparatus, and more particularly, to an improved particle trapping system in such electrical apparatus which utilizes a sublimable protective coating over an interiorlydisposed permanently sticky adhesive material.
High-voltage gas-insulated electrical apparatus, typical of which are transmission lines, circuit breakers, disconnect switches and the like, typically comprise an outer sheath or enclosure at low or ground potential, an inner electrode or electrodes at high potential disposed within the outer enclosure, and support insulators for insulatably supporting the electrode within the enclosure. An insulating gas is generally utilized to electrically insulate the electrode from the outer enclosure, with the result that the high dielectric strength of the insulating gas enables closer, more compact spacings between the electrode and the enclosure. Sulfur hexafluoride has been utilized as the insulating gas for many reasons associated with its useful combination of vapor pressure, chemical stability, cost, electric strength, thermal conductivity, and non-toxicity, among other properties.
One problem which has arisen in the design of these high voltage gas-insulated electrical apparatus, and more particularly with respect to the transmission lines, is the effect of undesired mobile conducting or semiconducting particles. These particles may be remaining in the line after assembly, or may be produced during operation, and can traverse between the outer enclosure and the inner electrode to thereby cause sparking, corona, or can lead to flashovers and breakdown of the insulating gas or the insulating support. These contamination particles can lower the breakdown strength of the sulfur hexafluoride gas. In order to overcome these particle effects, it may be necessary to increase the size of the gas-insulated transmission line, or include within the transmission line means for eliminating or deactivating the conducting particles.
One means utilized in the prior art to deactivate these particles has been the use of low electric field regions as taught by Trump in U.S. Pat. No. 3,515,939. The low field regions are created by including within the transmission line conducting electrodes which are electrically connected to the outer sheath and which have portions thereof spaced therefrom, so that a low field region is formed between the electrode and the outer sheath known as particle traps.
Another method utilized in the prior art is the so-called adhesive trap, in which particles become trapped on the adhesive surface, preferably in a location where the electric field is low. This type of adhesive trap was disclosed in U.S. Pat. No. 3,911,937 to Sletten et al.
Adhesive traps may be permanently sticky, or may be initially non-sticky. The permanently sticky adhesive trap involves coating the interior surface of the outer electrode with an adhesive material which is permanently adhesive. (As used throughout this specification, permanently adhesive means that the adhesive material is sticky throughout its lifetime, as contrasted to other adhesives which may be non-sticky at times.) The non-permanently sticky adhesive traps generally involve a surface film which again is deposited on the interior surface of the outer enclosure, but involves the surface film which is not sticky when deposited and only becomes sticky after the system is filled with the insulating gas, typical of which is sulfur hexafluoride, containing a solvent or softener such as cyclohexane or cyclehexene. This approach has recently run into difficulties in that the electric strength of the insulating gas is adversely affected. The permanently sticky adhesive trap also has problems, in that since the material is always sticky, difficulties in handling, assembling of the equipment, and the like are present.